1. Technical Field
The invention relates to tunable optical filters and, in particular, to optical filters which can be tuned by angular adjustment of a pivoting filter element.
2. Background
Tunable optical filters are used in, for example, wavelength-division multiplexing (WDM) optical communication system and tunable source applications. In one known tunable optical filter wavelength tuning is obtained by the rotating an interference filter in a collimated beam. In the 1550 nanometers wavelength region, 20 degree rotation of the filter typically shifts the passband wavelength by 40 nanometers. While such tunable optical filters feature simple design and stable operation, a disadvantage is that, as described by R. H. Eather et al. in Applied Optics, Vol. 8, No. 2, 1969, as incidence angle is increased, the passband of the filter shows polarization dependence, and the transmittance of the filter becomes polarization dependent. This occurs because the successive transmission and reflection between the layers of the interference filter element results in different losses for light beam components having different polarization.